1. Field of the Invention
The present invention relates to a waveguide-type acoustooptic device for guiding a light beam through an optical waveguide and diffracting the guided light beam with a surface elastic wave, and more particularly to a waveguide-type acoustooptic device which is prevented from suffering optical damage.
2. Description of the Prior Art
There is known an optical-waveguide-type light deflector as disclosed in Japanese Unexamined Patent Publication No. 61(1986)-183626, for example. The disclosed light deflector includes an optical waveguide made of a material which can propagate a surface elastic wave therethrough. A light beam which is guided through the optical waveguide is subjected to Bragg diffraction by a surface elastic wave which is generated in a direction transverse to the guided light beam. The angle through which the guided light beam is diffracted, i.e., the deflection angle, can continuously vary when the frequency of the surface elastic wave continuously varies.
A light spectrum analyzer, which has also been proposed heretofore as disclosed in U.S. Pat. No. 4,900,113, for example, similarly employs an optical waveguide for guiding a light beam. The guided light beam is diffracted by a surface elastic wave whose frequency is continuously changed. The wavelength of the guided light beam is measured on the basis of the frequency of the surface elastic wave when the diffraction of the light beam takes place.
According to another proposed light modulator as disclosed in U.S. Pat. Application Ser. No. 288,837, for example, a light beam guided through an optical waveguide is diffracted by a surface elastic wave. When the surface elastic wave is turned on and off, or intermittently generated, the diffracted light beam is also turned on and off, i.e., modulated by the surface elastic wave.
Each of the light deflector, the light spectrum analyzer, and the light modulator, as described above, is a waveguide-type acoustooptic device for diffracting a light beam guided through an optical waveguide, with a surface acoustic wave. Heretofore, LiNbO.sub.3 is widely used as the material of the substrate of the optical waveguide.
However, waveguide-type acoustooptic devices which have substrates of LiNbO.sub.3 are susceptible to optical damage. Optical damage, which is caused to an optical waveguide, tends to disturb the profile of a light wave or block the guiding of light when a highly intensive light beam is introduced into the optical waveguide. Therefore, the conventional waveguide-type acoustooptic devices with LiNbO.sub.3 substrates have been difficult to guide a highly intensive light beam. Particularly, the light modulator, referred to above, generally guides a light beam whose width has been reduced to a very small value, in order to increase the rate of modulation, i.e., shorten the time required for the surface acoustic wave to travel across the guided light beam. The optical waveguide of the light modulator is more liable to suffer optical damage because the reduced-width light beam applies intensified light energy to the optical waveguide.
It is known that an optical waveguide comprising an LiNbO.sub.3 substrate is less subject to optical damage if an introduced light beam is guided in the direction of a z-axis of the LiNbO.sub.3 crystal.
With the light beam guided in the z-axis direction, however, since effective refractive indexes for TE and TM modes are substantially the same as each other, when the light beam is introduced in the TE mode, it is diffracted without any mode conversion, namely, simply from the TE mode to the TE mode, and also with a mode conversion from the TE mode to the TM mode, a phenomenon referred to as the TE-TM mode conversion diffraction. Likewise, when the light beam is introduced in the TM mode, it is diffracted without any mode conversion, namely, simply from the TM mode to the TM mode, and also with a mode conversion from the TM mode to the TE mode. Therefore, the diffracted light beam is propagated in both TE and TM modes irrespective of the mode in which the light beam is introduced into the optical waveguide.
One way of emitting a light beam, which has been guided through an optical waveguide, out of the optical waveguide is to use a grating coupler on the surface of the optical waveguide for diffracting the guided light beam. A light beam which is guided in both TE and TM modes through the optical waveguide is diffracted through slightly different angles by such a grating coupler. Therefore, two light beams which travel slightly displaced light paths are emitted from the optical waveguide, and such two light beams cannot be focused into a small single spot. The two light beams thus emitted from the optical waveguide find little or no use at all.
If the diffracted light beam is directly emitted from the optical waveguide, such two light beams traveling along displaced light paths are not produced. Since, however, the light beam guided in both TM and TE modes is emitted from the optical waveguide, the emitted light beam contains components which are linearly polarized in two perpendicular directions. However, the light beam emitted from the optical waveguide should preferably be linearly polarized in only one direction in some applications. The light beam which contains two linearly polarized components, as described above, also find little or no use in such applications.